1. Field of the Invention
The present invention relates to a redundant path control apparatus which has logical disks, redundantly has paths to access the logical disks and provides a host computer with memory areas of the logical disks by adequately switching the paths, and a redundant path control method.
2. Description of the Related Art
A disk army system which can use a plurality of hard disk units as if they were a single hard disk unit is disclosed in, for example, Unexamined Japanese Patent Application KOKAI Publication No. 2002-304331. A host computer which constitutes the disk array system accesses data stored in logical disks constructed by a plurality of hard disk units. The access is carried out according to application software that is executed by the host computer.
The disk array system has plural paths to access individual logical disks, and controllers respectively associated with the paths. This can allow, for example, the host computer to make a fast access to the logical disks. Even when any path fails, data can be sent again using another path. That is, retry is possible using another path.
Further, the disk array system is capable of forcibly releasing a reserve state, when there is a logical disk which is in a state seized by a specific unit (reserved state), and switching the logical disk into a state seized by the host computer. Even when a retry process is terminated abnormally due to the seizure of the target logical disk by a specific device, therefore, the retry process can be executed again and successfully.
When maintenance work is performed on a controller in this disk array system, specifically, when the package of a controller or the firmware of a controller is to be replaced, maintenance work is carried out according to one of the following three schemes (1) to (3) in order to prevent input/output accesses to the controller targeted for maintenance work.
(1) Maintenance work is performed after all the accesses to the disk array from the host computer are stopped.
(2) Over a given period of time in which maintenance work on a controller is completed, the controller is unresponsive to an I/O access from the host computer according to application software. When the controller becomes unresponsive, the host computer decides that some kind of fault has occurred in a path to access the logical disk that has been used so far, and resumes the access by switching the path to an adequate one. This cancels an access to the controller targeted for maintenance work.
(3) Maintenance work is performed after the host computer changes a path to access the target logical disk as a consequence of a manual operation performed on the host computer.
The schemes (1) to (3) face the following problems.
The scheme (1) stops every access to the disk array from the host computer. This essentially requires that the process of the application software running on the host computer should be terminated.
According to the scheme (2), the controller that is undergoing maintenance work does not respond to an input/output request from the host computer, so that an access by the host computer would fail. However, a retry process using a substitute path (i.e., another controller) is performed. It is therefore unnecessary to terminate the process of application software. The host computer cannot however discriminate whether the cause for the access failure is originated from a fault in the path or the controller undergoing maintenance work, and treats both cases as having originated from a fault in the path. When the cause for an access failure cannot be discriminated, maintenance work is likely to be in confusion.
According to the scheme (3), a path to access a logical disk is manually changed beforehand so that an access request to the target controller for maintenance work will not be generated (i.e., the path is changed in such a way as to go through a controller which is not targeted for maintenance work). When the number of host computers to be connected to the disk array system and/or the number of paths to be connected to each host computer becomes larger, the content of the manual operation becomes complicated, increasing a possible risk of erroneously switching the path.